1. Field of the Invention
Exemplary embodiments of the present invention relate to a log periodic antenna; and, more particularly, to a log periodic antenna having a reduced beam width of the H-surface radiation pattern and high-gain directivity.
2. Description of Related Art
In general, an antenna is configured to convert electric signals, which are described in terms of voltage/current, into electromagnetic waves, which are described in terms of electric/magnetic fields, and vice versa. Antennas include dipole antennas, monopole antennas, patch antennas, horn antennas, parabolic antennas, helical antennas, slot antennas, log periodic antennas, etc.
The log periodic antennas have broadband characteristics and a suitable level of gain, and thus are widely used for TV reception or communication. The type of broadcasting and communication services has recently become more diversified, such as IMT-2000, wireless LAN, portable wireless Internet, etc. As a result, there is an increasing demand for antennas capable of covering broadband, dual-band, triple-band, etc, and the availability of log periodic antennas is also increasing in this connection.
The log periodic antennas are classified, according to the type of repeated structure, toothed planar antennas, toothed trapezoid antennas, trapezoid wire antennas, and zigzag wire antennas. Among the log periodic antennas of various shapes, log periodic dipole antennas having an array of planar or wired dipoles are widely used.
A typical broadband log periodic dipole antenna includes a series of serially-fed dipole radiation elements, and its design parameters include the geometric ratio of the log periodic structure (τ), spacing factor (σ), and the length (λ/2) of a single dipole antenna of a specific band. Therefore, any attempt to reduce the length of the dipole radiation elements and the overall size is limited. In other words, higher gain may be obtained by increasing the geometric ratio of the log periodic structure (τ) and spacing factor (σ), but the length of the antenna boom and the number of radiation elements inevitably increase, making the overall antenna size bigger.
Recent wireless communication systems have a tendency towards broadband characteristics or smaller sizes. This means that element development is directed to reducing the overall antenna size while maintaining broadband characteristics.
In an attempt to solve the above-mentioned problem, it has been proposed to replace the dipole radiation elements of a log periodic dipole antenna with loop elements so that the element length is reduced. It has also been proposed to bend the end of dipole radiation elements, or employ size-reduced or foreshortened dipoles.
These approaches may reduce the length of dipole radiation elements, but cannot increase the gain. Therefore, log periodic antennas having a small beam width and good directivity, which are applicable to wireless communication systems, must come in a different type.
In the case of a wireless communication system where an antenna is moved to measure the strength of received signals and find the direction from which radio waves are transmitted, specifically a portable direction finding system, a conventional log periodic dipole antenna is usually employed. This has problems in that the overall antenna size is only large in the two-dimensional plane, and the 3 dB beam width of the H-surface radiation pattern is as large as 120°, making signal direction finding unreliable. Therefore, improvement of directivity based on high-gain structure, combined with the trend towards broadband characteristics and small sizes of log periodic dipole antennas, is a prerequisite for higher direction finding accuracy of direction finding systems.
Consequently, it is requested to develop a log periodic antenna having a small beam width and high gain while maintaining the broadband characteristics of conventional log periodic antennas.